Polytetrafluoroethylene (PTFE) is a tough, relatively inert, linear organic polymer of the monomer, C.sub.2 F.sub.4 or tetrafluoroethylene (TFE), which monomer has the following formula: ##STR1##
The carbon atoms of the monomer have a valence of four and the fluorine atoms have a valence of one. When the atoms combine to form a C.sub.2 F.sub.4 molecule, each atom acquires a complete outer shell of eight electrons. This combination of atoms results in a particularly stable, covalently bonded compound. Indeed, the carbon-fluorine bond is among the strongest known among organic compounds.
When TFE is polymerized to become the highly crystalline long chain polymer PTFE, the same stable electron configuration exists. Its stability is manifested by its chemical inertness, lubricity and unusually high melting temperature. It also has the lowest coefficient of friction of any known solid: 0.06 against polished steel. It also has one of the highest melting temperatures of any known polymer, 327.degree. C., and is stable within a broad temperature range of about -250.degree. C. to 260.degree. C.
The chemical stability of PTFE makes its adhesion difficult. A PTFE surface is highly non-polar because of strong interatomic affinity. The surface atoms are electrically satisfied, so that the substrate has an unusually low surface energy of 25 mJ/m.sup.2 at 25.degree. C. Matter with such a low surface energy is not wettable and is difficult to bond in that state.
In some applications, PTFE can be mechanically bonded to the mating parts, but this method can be complex and expensive. A more practical method is to form an adhesive bond to the PTFE. Such a bond can be made possible by chemically treating the PTFE to enhance bonding power. Chemical processes can involve chemical and physical changes in a thin surface layer of about 100 A.degree. to 100 .mu.m of PTFE without affecting the bulk properties of the material.
One type of process to change the surface layer of PTFE is etching by which a corrosive chemical agent erodes the surface of a solid. Sodium Liquid Etching (SLE) is an established and successful technique for preparing PTFE surfaces prior to bonding. Sodium etching causes defluorination of the PTFE resulting in surface unsaturations which enhance bonding. According to this process, sodium metal chips are dissolved in a solution of ammonia, or tetrahydrofuran and naphthalene. The PTFE sample is then submerged in the solution and is allowed to react for 1-5 minutes etching a layer in the surface of the PTFE of about 1 .mu.m. This has been shown to cause dramatic increases in surface energy, polarity, wettability and bondability of the PTFE. The new polarity increases the wettability and bonding power of PTFE. However, this process is very dangerous to use due to the highly toxic and flammable chemicals existing in the etchant solution and the large amounts of sodium necessary for the reaction.
The problems inherent in the SLE treatment of PTFE have left a need for a new method to alter the chemistry of the surface of PTFE to improve its bonding cabability.